A major hurdle towards the implementation of nanomaterials in practical electronic and optoelectronic device architectures is the tradeoff between the need to minimize charge transport lengths (electronic and/or ionic) vs. the need to achieve high mass loadings of active material per geometric device area (to obtain high capacity or high optical density). Controlling the micro and nanoscale structure of a transparent conducting oxide (TCO) electrode could yield an excellent platform for modern and future optoelectronic devices by combining large interfacial areas and high loading of active material with short charge transport distances and desirable light management characteristics. These are key features for boosting the performance of many devices including electrochemical sensors, catalysts, supercapacitors, batteries, photovoltaics (PV) and photoelectrodes. When used as a support for electrocatalyst and photoelectrode materials (e.g. for solar fuel synthesis), large-pore transparent conductive high surface area electrodes (HSEs) can be an enabling technology at both the fundamental and applied research levels. In electrocatalytic applications, large interfacial areas permit more efficient charge transfer by lowering local current densities which minimizes overpotential losses, while larger pores minimize mass transport losses that would otherwise arise from the extensive tortuosity that is characteristic of typical high surface area architectures. The latter is particularly important in applications where device efficiency is coupled to mass transport of liquid or gas phase species, thus necessitating the ability to tune and optimize HSE pore sizes to enable facile diffusion of reactants and products (such as gas bubbles) throughout the structure. Achieving an optimized structure enables higher turnover frequencies per geometric electrode area, resulting in more efficient product generation or more sensitive screening. In optoelectronic applications, the benefits of HSEs are two-fold: (1) increased loading of active materials such as thin film absorbers allows for increased device optical density while maintaining short minority carrier path lengths through the absorber and (2) texturing of the surface imparts more efficient light management than in a comparable planar device. Together, these benefits can potentially enable higher overall device efficiency.
HSEs, including those made of ITO, are characterized by three fundamental performance metrics which must be maximized simultaneously: (1) surface area, where the figure of merit is roughness factor (RF), (2) visible transparency and light management within the active layer, and (3) electrical conductivity. Additional desirable properties include mechanical strength (cohesion and adhesion), chemical and electrochemical stability, thermal stability and a facile synthesis amenable to large area fabrication. Historically, research directions for TCO films, including ITO, focused on creating dense, highly transparent planar films for commercial applications in consumer electronics, thermal glass coatings and other areas in which structured, light scattering films such as the HSE described here would be undesirable. The shifting of recent work towards structured films reflects a growing need for new materials systems related to high surface area TCO electrodes. Nevertheless, current literature covering structured TCO HSE materials is limited.
The structured ITO can be divided into two types of syntheses: (1) with or (2) without the use of templating/structure-directing agents. One of the most crystalline and highest electronic quality material reported among structured ITO films was prepared free of such agents but employed a costly high vacuum electron-beam synthesis step. Other examples of template-free structured ITO films include free-standing fibrous mats which may exhibit fairly high surface area but require further processing for incorporation into planar devices, as well as fiber networks. The majority of high surface area ITO syntheses require the addition (and inevitable removal) of templating/structure-directing agents, which can add significant cost and complexity that may limit the feasibility of large scale manufacturing. These include the use of (block co-)polymers, polymer spheres or binders, and densifying agents to produce structured ITO thin films and powders with periodic arrays of pores having either mesoscale (2-50 nm) or macroscale (>50 nm) diameters, as well as disordered ITO particle aggregations. The only report of an ITO electrode with a quantified surface area is an ordered mesoporous ITO thin film with ˜10 nm pore diameters and a roughness factor of 45±3. This film has also demonstrated efficacy as an electrode, and thus represents the highest-performance high surface area ITO electrode to date.
What is needed is a method of fabricating transparent conducting HSE's with tunable roughness factors and adjustable pore sizes from the nanometer to micron scale using a low-cost and reduced complexity process that does not use templating.